Steel industries, having large facilities and large amounts of energy consumption, are under necessity of a continuous process, for the purpose of process-saving and energy-saving. In a field of seamless pipes, for example, a technology that heat treatment, such as “quenching” and “tempering”, which was so far provided by a facility of another line is continuously carried out after rolling process, is under consideration.
It is necessary to carefully select process conditions in order to materialize the continuous process, since the seamless pipes have extremely severe demands for reliability of the products. The following present applicants disclose some process conditions in terms of energy-saving.
[Patent document 1] Republished patent application WO 1996/12574-B
[Patent document 2] Japanese Unexamined Patent Publication No. 1996-311551-A
[Patent document 3] Japanese Unexamined Patent Publication No. 2001-240913-A
In recent years, excellent performance has been required for the seamless pipes. At a relatively high rolling finishing temperature as described in the above-mentioned documents, however, it becomes apparent that crystal grain is still coarse even when subsequent concurrent heating and heat treatment is conducted and that it is difficult to cope with higher demands especially concerning toughness of the products.